Waves that move in a vacuum, being in matter’s vacuum or in total vacuum, do not encounter obstacles. Waves will stop only when they hit a proton or an electron, taking up little space in a vacuum.

Also liquid helium becomes superfluid at +2.7° Kelvin. Heike Kamerlinghen, who is the first to liquefy helium, sees that helium’s superfluidity means that it does not oppose any resistance to objects going through it.

Uncharged particles called neutrinos can also be detected using photomultipliers. Neutrinos go through matter. Larger than waves, neutrinos are detected when they destroy each other. For example, there is the Super-Kamiokande neutrino detector project in Japan.